Abstract: Prostate cancer (PCa) is a remarkably adaptive disease. First line therapy for PCa is androgen deprivation. However, resistance invariably emerges, resulting in a lethal phase termed castration-resistant PCa (CRPC). Even with the profound AR-targeting achieved by current standard of care agents abiraterone and enzalutamide, CRPC remains incurable. CRPC is characterized by multiple compensatory signaling mechanisms including reciprocal activation of PI3K/Akt/mTOR signaling and AR-ErbB receptor cross-talk. Notably, these pathways converge on the signaling networks, feedback loops, and cellular mechanisms that mediate oncogenic lipid metabolism, which is now recognized as a central driver of CRPC growth and progression. Meaningful improvement in anti-tumor efficacy is likely to require novel strategies that simultaneously target the AR axis and the network of compensatory signaling pathways on which CRPC depends. We have identified Sigma1 as a multi-functional scaffolding protein that is aberrantly expressed in PCa and that it is required for PCa cell growth proliferation. Sigma1 allosterically modulates cancer-specific associated proteins involved in driving oncogenic lipid metabolism, including AR and ErbB receptors. Sigma1 also regulates cellular lipid and protein homeostasis pathways, and plays a critical role in supporting the increased demand for lipid and protein synthesis associated with tumor growth. We have developed a series of novel small molecule inhibitors of Sigma1 that disrupt lipid homeostasis and induce targeted degradation of AR and ErbB receptors in PCa cells, resulting in inhibition of PCa growth in vitro and in vivo with minimal toxicity to normal cells. The overarching problem addressed in this proposal is how to target the critical mechanisms by which lethal CRPC becomes resistant to AR-targeted therapy. We hypothesize that Sigma1 serves as a multifunctional nexus between oncogenic driver proteins and lipid metabolism in PCa, such that Sigma1 inhibition disrupts not only key drivers of tumor growth and lipid metabolism (AR, ErbB), but also inhibit their downstream and convergent pathways. In Aim 1 we will define a novel Sigma1-AR-ErbB/PI3K/mTOR-lipid metabolism pathway and feedback loop that engages ErbB/PI3K signaling in CRPC. We will show that the anti-tumor efficacy of Sigma1 inhibitors in PCa is due to suppression of this pathway as well as disruption of key convergent and complementary cellular processes critical for PCa growth fueled by lipid metabolism. In Aim 2 we will demonstrate the efficacy of Sigma1 inhibition in a cohort of patient derived xenograft (PDX) models that encompass the genotypic and phenotypic heterogeneity of CRPC, using in vitro organoid and in vivo tumor models. Inhibition of Sigma1 in PCa represents a novel therapeutic approach that targets multiple, interdependent mechanisms involved in CRPC progression and development of resistance, and it provides a rational basis for designing vertical and horizontal combination treatment strategies to block the enhanced lipid metabolism that fuels lethal, treatment-refractory CRPC.